In this nested case-control study in mild-to-moderate COVID-19 outpatients, the concentration of the biomarkers CRP, IL6, IP10, ferritin, IFNα, IL8, IL1RA, MCP1, and RANTES determined within 7 days of symptoms onset showed good individual prognostic performance for progression towards hospitalization due to COVID-19 within 28 days. This evidence concerns the gene CXCL10 and COVID-19.